Saturday, March 7, 2015

One often sees descriptions of speleothems implying that water evaporation is the single important process causing calcium carbonate (CaCO3- calcite or aragonite) to precipitate. See, for example, page 26 of the photographic tour I've linked a number of times now. However, page 6 of that same tour also mentions another cause: loss of dissolved CO2. When CO2 goes into aqueous solution, it makes carbonic acid, which dissolves calcite. When the CO2 leaves solution, the solution becomes over saturated with calcium, which then precipitates out as calcite. The common presumption is that water coming into the cave has received most of its CO2 load as rain falling through the air- and indeed, when it hits the ground, rainwater is slightly acid, typically with a pH of about 5.5. What's rarely pointed out is that soil CO2 levels are extraordinarily high compared to freely-mixing atmospheric air: tens to over a hundred times higher. From here (and the following is only a third of the full diagram, which has lots more info):

So when rain hits the ground, it's in equilibrium with an atmospheric partial pressure of CO2 of (as of present day) 0.04%. Then it seeps into soil with a partial pressure of CO2 of 2%-5%, or 50 to 125 times that of the atmosphere! In a very real sense, the waters' passage through soil supercharges its acidity, allowing to be a much more powerful dissolving and erosive agent. Cave air, on the other hand, mixes more freely with the atmosphere, so tends to be much closer to "normal" atmospheric conditions. When ground water reemerges into the cave, it fairly quickly re-equilibrates to the conditions therein. Agitation, such as falling as water droplets, accelerates the degassing of CO2.

Now all this is not to say water evaporation plays no role in the creation of speleothems. In some situations it may indeed be the major player. My experience, though, is that the role of soil CO2 gets much less attention than it probably should. I don't think most people, even geoscientists, realize how high concentrations of that gas can get in natural conditions just under their feet.

Photo unmodified. May 9, 2013. FlashEarth Location. (Since we're underground, I have only a vague idea where this is with respect to the surface.)